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In the C programming language, restrict is a keyword, introduced by the C99 standard, [1] that can be used in pointer declarations. By adding this type qualifier , a programmer hints to the compiler that for the lifetime of the pointer, no other pointer will be used to access the object to which it points.
Restrict the scope of data to the smallest possible. Check the return value of all non-void functions, or cast to void to indicate the return value is useless. Use the preprocessor sparingly. Limit pointer use to a single dereference, and do not use function pointers.
As of 2014 and C11, there are four type qualifiers in standard C: const , volatile , restrict and _Atomic – the latter has a private name to avoid clashing with user-defined names. [3] The first two of these, const and volatile , are also present in C++, and are the only type qualifiers in C++.
1 Why restrict on all pointers? 2 Alternative working example. 1 comment. 3 "restrict" optional in C++ [but not in C] an important difference? 1 comment.
The element pc requires ten blocks of memory of the size of pointer to char (usually 40 or 80 bytes on common platforms), but element pa is only one pointer (size 4 or 8 bytes), and the data it refers to is an array of ten bytes (sizeof * pa == 10).
Specifically, C allows a void* pointer to be assigned to any pointer type without a cast, while C++ does not; this idiom appears often in C code using malloc memory allocation, [9] or in the passing of context pointers to the POSIX pthreads API, and other frameworks involving callbacks. For example, the following is valid in C but not C++:
This kind of type punning is more dangerous than most. Whereas the former example relied only on guarantees made by the C programming language about structure layout and pointer convertibility, the latter example relies on assumptions about a particular system's hardware.
This is an accepted version of this page This is the latest accepted revision, reviewed on 9 February 2025. There is 1 pending revision awaiting review. General-purpose programming language "C programming language" redirects here. For the book, see The C Programming Language. Not to be confused with C++ or C#. C Logotype used on the cover of the first edition of The C Programming Language ...